1. Field of the Invention
The present invention relates to a transmission control device for an automatic transmission that changes the speed of rotation output from an engine equipped with a turbocharger, and particularly, to a transmission control device for an automatic transmission that suppresses a reduction in fuel efficiency and improves the state of exhaust gas.
2. Description of the Related Art
For example, a diesel engine mounted in a vehicle, such as a passenger car, increases the amount of intake air by performing a turbocharging operation using a turbocharger or the like so as to set the vehicle in a lean fuel state during a normal running mode, thereby preventing the occurrence of particulate matter (PM), such as soot, which can occur due to a locally rich fuel state within a combustion chamber.
However, in a diesel engine in the related art, when the boost pressure is low, as in a low rotation period, and the amount of intake air is small, oxygen becomes insufficient during a high load period in which the amount of fuel injection increases, sometimes resulting in an increase in the amount of PM emission.
In contrast, increasing the frequency of use of a high rotation range of the engine by increasing the reduction gear ratio of a transmission can make it easier to ensure a sufficient amount of intake air in the engine. Although this may be advantageous in terms of a countermeasure against exhaust gas, frequent use of the high rotation range in this manner can lead to a reduction in fuel efficiency due to an increase in friction loss.
As an example of a related art technology that relates to transmission control performed in view of improving the state of exhaust gas of a diesel engine, Japanese Unexamined Patent Application Publication No. 2004-183675 discloses a technology of detecting an operational state in which black smoke tends to occur, such as when the vehicle is running at a high altitude, and controlling an automatic transmission within a low gear range.
However, even in a condition where there is a relatively low tendency for black smoke to occur, as in a standard atmospheric state at, for example, a low altitude, there is sometimes a time response lag until a target boost pressure is obtained in a transient operational state, such as starting and accelerating the vehicle from an idled state. This can result in an inability to achieve an appropriate operational state in terms of both exhaust gas and fuel efficiency.